The present invention relates generally to a technique for compensating for image retention in an imaging detector, such as those used in digital x-ray imaging systems. More particularly, the invention relates to compensation of image retention through prediction of decay of a ghost image produced by an earlier image in a sequence, and for enhancing later images in the sequence based upon the predicted decay.
Digital x-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital x-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient""s tissues similar to those available through conventional photographic film techniques.
Digital x-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films were prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital x-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
Despite their utility in capturing, storing and transmitting image data, digital x-ray systems are still overcoming a number of challenges. For example, x-ray systems may be employed for a range of different types of examination, including radiographic and fluoroscopic imaging. Among other distinctions, these two types of imaging examinations are characterized by significantly different radiation levels used to generate the image data. Specifically, radiolographic imaging sequences employ substantially higher radiation levels than fluoroscopic imaging sequences. In a number of applications, it may be desirable to perform both types of imaging sequences sequentially to obtain different types of data and to subject patients to lower overall radiation levels. However, current digital x-ray systems may encounter difficulties in performing fluoroscopic imaging sequences following radiological sequences.
Specifically, current digital x-ray systems employ amorphous silicon detectors with arrays of photodiodes and thin film transistors which beneath an x-ray scintillator. Incident x-rays interact with the scintillator to emit light photons which are absorbed by the photodiodes, creating electron-hole pairs. The diodes, which are initially charged with several volts of reverse bias, are thereby discharged in proportion to the intensity of the x-ray illumination. The thin film transistor switches associated with the diodes are then activated sequentially, and the diodes are recharged through charge sensitive circuitry, with the charge needed for this process being measured.
Raw signals from the detector may require several corrections to yield an accurate measure of the incident x-ray intensity. One of these corrections is for offset, or the signal which exists in the absence of x-ray illumination. One source of this offset is leakage current in the diodes. Another source of offset in current digital x-ray detectors is related to the previous history of illumination of the diodes. Due to the nature of the amorphous silicon of the detector panel, the photodiodes contain traps which are filled after x-ray excitation, and which thereafter empty in a decay process with a relatively long time constant. As a result, a decaying image is retained by the detector.
The magnitude of image retention in x-ray detectors is relatively small, and decays with time as the traps empty thermally. In single-shot radiographic applications, image retention does not generally cause problems because a relatively long period of time exists between exposures. An offset image for correction of a subsequent exposure is generally feasible fairly close in time prior to the exposure. In the latter mode of operation, the error in the image after offset correction is equal to the difference in the retained image between the time of the x-ray image and the time of the offset image. If sufficient time has elapsed between the previous x-ray exposure, this difference is minimal.
In fluoroscopic imaging, on the other hand, an accurate representation of the offset can be obtained by reading the detector continually between x-ray exposure periods, and averaging these offset images. The averaged offset image can be frozen at the start of x-ray activation, and used to correct the x-ray images. As long as the x-ray exposure interval is not too long, the retained signal in the offset image will closely approximate that in the x-ray images, and the error due to the retained signal will be small in the corrected images.
Image retention in x-ray detectors poses a substantial problem, however, in applications requiring mixed radiographic and fluoroscopic operation. Again, because the fluoroscopic signal levels are substantially lower (eg. two to three orders of magnitude smaller) than the radiographic signals, when a flueroscopic imaging sequence follows a radiographic exposure, the retained image, although a small fraction of the radiographic signal, can be comparable to or even larger than the fluoroscopic signal. If uncorrected, a ghost of the radiographic image will appear in the reconstructed fluoroscopic image.
There is a need, therefore, for an improved technique for compensating for retained images in discrete pixel image detectors. There is a particular need for a technique which can be applied to digital x-ray systems to compensate for image retention in sequential imaging exposures, such as in fluoroscopic exposures following radiographic exposures.
The present invention provides a technique designed to respond to these needs. The invention is particularly well suited to the specific application of compensating for image retention following radiographic exposures in digital x-ray systems. However, the technique may be advantageously employed in other domains, including within and outside the medical diagnostic imaging field, where appropriate. Moreover, the present technique may be employed in both existing systems, as well as in new or future digital imaging systems, particularly those employing amorphous silicon detectors. Because the technique is based upon sampling of data from the detector, and processing the sampled data in accordance with a computer-implemented routine, it is susceptible to use in imaging systems both in their basic control algorithms, as well as in patches or enhancements to existing control or signal processing software.
The technique is based upon a sampling of image data during a period following a first exposure or examination. The sampled data represents values for individual pixel regions of an image matrix. A plurality of sampled data sets is preferably acquired over time. The time period for acquisition of the data may generally be a fixed sampling interval of the detector and its associated control circuitry. Based upon the sampled data, the decay of the retained image is characterized and predicted based upon a prediction model. Predicted values of the decaying retained image are then used to correct or compensate for any remaining retained image which may be present through all or part of a subsequent imaging exposure or examination.